The automatic beatmatching in Mashup works well for most songs, but occasionally a slight adjustment can make things even tighter. If the beatmatching is completely off, that indicates that the BPM value on the track controls is wrong. If the BPM on the track controls is not exactly the BPM of the track at its original speed beatmatching will not work. Try entering a more accurate BPM if it is completely incorrect, or rounding the BPM on the track controls to a whole number. For example, if you track A that is reported in Mashup as 127.89, and track B that is reported as 126.78, try entering 128 for track A and 127 for track B. This will fix most issues with beatmatching, but if things still sound a little off, try making a phase adjustment using the Phase buttons on the track controls.
Will you please let us know if that helps?